Method for converting an optical wavelength using a monolithic wavelength converter assembly

ABSTRACT

A method of converting an optical wavelength includes providing a wavelength converter assembly with a photodetector and a laser that have a common epitaxial structure with areas of differing bandgap. The laser including a laser resonator. An optical input with a first wavelength is absorbed at the wavelength converter assembly. A first electrical signal is generated from the photodetector in response to the optical input. The first electrical signal is conditioned and produces a conditioned first electrical signal. A second electrical signal is generated from the conditioned first electrical signal. A laser output is generated from a gain medium of the laser at a second wavelength in response to the second electrical signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation in part and claims the benefit ofpriority of U.S. Provisional Application Ser. No. 60/152,072, filed Sep.2, 1999, U.S. Provisional Application Ser. No. 60/152,049, filed Sep. 2,1999, U.S. Provisional Application Ser. No. 60/152,038, filed Sep. 2,1999, which applications are fully incorporated by reference herein.This application is also continuation-in-part Ser. No. 09/614,377 filedJul. 12, 2000 which claims benefit of No. 60/152 049 filed Sep. 2, 1999and claims benefit of Prov. No. 60/152,038 filed Sep. 2, 1999 and acon't-in-part of U.S. Ser. Nos. 09/614,665, 09/614,865, 09/614,378,09/614,376, 09/614,674, 09/614,195, 09/614,375, and 09/614,224, allfiled on the same date as this application Ser. No. Jul. 12, 2000, whichapplications are fully incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to wavelength converters of the type desirable incertain wavelength division multiplexed optical communication networks,as well as other applications where it is desirable to change thewavelength of the optical carrier of a modulated lightwave, and moreparticularly to optoelectronic wavelength converters in which anincoming lightwave having a first wavelength is detected by aphotodetector that produces an electrical signal that in turn modulatesa source of an outgoing lightwave having a desired second wavelength.

2. Description of the Related Art

Optoelectronic wavelength conversion processes have used as separatephotodetectors, receiver and regeneration circuits, transmitter anddriver circuits, and directly or externally modulated lasers. S. J. B.Yoo, “Wavelength conversion technologies for WDM network applications,”J. Lightwave Techn. 14 (6) (June, 1996). These discrete-componentwavelength converters have tended to be relatively bulky and expensiveto manufacture. Also, the lasers generally have a fixed wavelength or avery limited tuning range.

There is a need for a monolithic wavelength converter assembly thatprovides for the process of detection and regeneration at some otherwavelength. There is a need for a monolithic wavelength converterassembly fabricated on one semiconductor substrate using compatiblephotonic integrated circuit technology for all components. There is afurther need for a wavelength converter assembly where signalamplification is obtained without the use of electronic transistors.There is yet a further need for a wavelength converter assembly whereconditioning of the signal is done in combination with the detection ormodulation process in the optical or electrical domain. There is still afurther need for a wavelength converter assembly that has a wide tuningrange and all of the components are fabricated on one semiconductorsubstrate using compatible photonic integrated circuit technology.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of one embodiment of a wavelength converterassembly of the present invention.

FIG. 2(a) is a top down schematic view of a waveguide photodetector thatcan be part of the FIG. 1 wavelength converter assembly.

FIG. 2(b) is a top down schematic view of a waveguide photodetector thatcan be part of the FIG. 1 wavelength converter assembly.

FIG. 2(c) is a top down schematic view of waveguide photodetectorintegrated with a semiconductor-optical amplifier (“SOA”) preamplifierthat can be part of the FIG. 1 wavelength converter assembly.

FIG. 2(d) is a top down schematic view of a waveguide photodetectorintegrated with a SOA preamplifier and a tunable resonant-cavity filterthat can be part of the FIG. 1 wavelength converter assembly.

FIG. 3(a) is a cross sectional view of the semiconductor layer structureof the FIG. 2(d) assembly in which passive sections are created byremoval of the active regions prior to regrowth.

FIG. 3(b) is a cross sectional view of the semiconductor layer structureof the FIG. 2(d) assembly in which passive sections are created byvariable thickness and composition quantum-wells via intermixing afteruniform growth or selective area growth.

FIG. 4(a) is a schematic top down view of a sampled-gratingdistributed-Bragg-reflector (“SGDBR”) tunable laser having aseries-connected, axially segmented multiple-active region that can bepart of the FIG. 1 wavelength converter assembly.

FIG. 4(b) is a schematic of a SGDBR tunable laser that has aseries-connected, vertically stacked multiple-active region that can bepart of the FIG. 1 wavelength converter assembly.

FIG. 4(c) is a schematic top down view of a SGDBR tunable laser with anintegrated external SOA that can be part of the FIG. 1 wavelengthconverter assembly.

FIG. 4(d) is a schematic top down view of a SGDBR tunable laser with anintegrated external electro-absorption modulator (EAM) and two SOAs thatcan be part of the FIG. 1 wavelength converter assembly.

FIG. 5(a) is a cross sectional view of the FIG. 4(a) structure.

FIG. 5(b) is a cross sectional view of the FIG. 4(b) structure.

FIG. 6 is schematic diagram of an equivalent circuit that can be usedwith the structures of FIGS. 2(a), 2(b), 4(a) and 4(b) as well as anintegrable current conditioning circuit.

FIG. 7 is a plot of the desired impedance of the FIG. 1 nonlinearcurrent conditioning circuit.

FIGS. 8(a) and (b) illustrate an embodiment of a monolithic wavelengthconverter assembly of the present invention where the photodetector isintegrated directly on top of the laser.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide animproved wavelength converter assembly.

Another object of the present invention is to provide monolithicwavelength converter assembly that provides for the process of detectionand regeneration at some other wavelength.

A further object of the present invention is to provide a monolithicwavelength converter assembly fabricated on one semiconductor substrateusing compatible photonic integrated circuit technology for allcomponents.

Yet another object of the present invention is to provide a wavelengthconverter assembly where signal amplification is obtained without theuse of electronic transistors.

Another object of the present invention is to provide a wavelengthconverter assembly where conditioning of the signal is done incombination with the detection or modulation process in the optical orelectrical domain.

A further object of the present invention is to provide a wavelengthconverter assembly that has a wide tuning range and all of thecomponents are fabricated on one semiconductor substrate usingcompatible photonic integrated circuit technology.

Yet a further object of the present invention is to provide a monolithicwavelength converter assembly that provides high data bandwidths.

Another object of the present invention is to provide a monolithicwavelength converter assembly that provides a large output opticalsignal amplitude without the need for integrated transistors forelectronic amplification.

Still a further object of the present invention is to provide amonolithic wavelength converter assembly that provides conditionedoutput data waveforms with lower noise and distortion than at an input.

Another object of the present invention is to provide a monolithicwavelength converter assembly that can be extended to large arrays ofwavelength converters integrated on one substrate with photonicintegrated circuit technology.

These and other objects of the present invention are achieved in awavelength converter assembly that includes a substrate. An epitaxialstructure is formed on the substrate with areas of different opticalproperties. A laser and a photodetector are formed in the epitaxialstructure. The photodetector generates a first electrical signal inresponse to an optical signal. A conditioning circuit is coupled to thelaser and the photodetector. The conditioning circuit receives the firstelectrical signal and provides a second electrical signal to the laserto modulate its optical output.

In another embodiment of the present invention, a wavelength converterassembly includes first and second semiconductor layers formed in anepitaxial structure. The first and second semiconductor layers havingdifferent dopings. A first waveguide layer is formed between the firstand second semiconductor layers. The first waveguide layer includesfirst and second reflectors that define a resonant cavity. An opticallyactive gain medium is disposed between the first and second reflectors.A photodetector is formed on the first semiconductor layer and includesan optically active absorber region. The photodetector generates a firstelectrical signal in response to an optical input.

DETAILED DESCRIPTION

Referring now to FIG. 1, a wavelength converter assembly 10 of thepresent invention provides for the process of detection and regenerationat some other wavelength to be carried out with a monolithic apparatus.Wavelength converter assembly 10 is fabricated on one semiconductorsubstrate using compatible photonic integrated circuit (IC) technologyfor all components. An advantage of wavelength converter assembly 10over other devices is that signal amplification is obtained without theuse of electronic transistors, which would involve incompatiblefabrication technology. Moreover, the conditioning of the signal may bedone in combination with the detection or modulation process in theoptical or electrical domain. Laser output from wavelength converterassembly 10 can have a wide tuning range so that a large number ofoutput wavelengths are possible.

In one embodiment, the elements of wavelength converter assembly 10 arefabricated on a single wafer. The various elements are derived from acommon epitaxial layer structure, and are fabricated by common processsteps.

Monolithic integration of optically dissimilar elements is accomplishedby a method of fabrication that tailors optical properties of selectedregions to a desired electro-optic function. Tailored opticalproperties, including the band gap, result in optically active andpassive regions on the same wafer beginning from a common epitaxiallayer structure. Further, the common fabrication process steps requiredfor forming the apparatus elements are compatible with photonic devicefabrication processes presently used in the lightwave industry. Thus,wavelength converter assembly 10 is readily manufacturable.

In a particular embodiment, the fabrication methods to selectivelytailor the band gaps of regions of the wafer of wavelength converterassembly 10 include the steps of, implantation of impurities by lowenergy ions (less than about 200 eV) in a portion of a selected waferregion near the wafer surface; and annealing the wafer. This allows theimpurities and vacancies implanted near the wafer surface to diffusethroughout the selected region and tailor the region's band gap to adesired electro-optic function.

For example, in the passive waveguide regions of the phase shift andmirror sections of a tunable laser element 12, the effective bandgapshould be somewhat larger (e.g., >0.1 eV) than the operating lightwaveenergy, which is only slightly larger (typically ˜0.01-0.05 eV) than theeffective bandgap of the active layers in the gain section. Integratedexternal modulator elements may have sections with the same largerbandgap as the other passive regions, or a bandgap intermediate betweenthat of the active and passive sections for some desired functionalitysuch as chirp reduction or improved-linearity. Integrated externalamplifier elements (M. J. O'Mahony, “Semiconductor laser OpticalAmplifiers for Use in Future Fiber Systems,” J. Lightwave Techn. 6 (4)(April, 1988.); A. E. Kelly, I. F. Lealman, L. J. Rivers, S. D. Perrin,and M. Silver, “Low noise figure (7.2 dB) and high gain (29 dB)semiconductor optical amplifier with a single layer AR coating,”Electron. Lett., 33 pp 536-8 (1997.)) may have the same bandgap as theactive gain section or a slightly modified bandgap for somefunctionality, such as increased saturation power or improved chirp ofmodulator/amplifier combinations. Robert G. Walker, “High-Speed III-VSemiconductor Intensity Modulators,” IEEE J Quant. Electron., 27, (3),654-667, (March 1991); F. Koyama and K. Iga, “Frequency Chirping inExternal Modulators,” J. Lightwave Tech., 6 (1), 87-93, (January 1998).

In various embodiments of the present invention, the passive regions arecreated by selective removal of the lowest bandgap layers responsiblefor gain in the active regions within the same sequence as some otherprocessing steps, such as grating formation in the mirror regions, arebeing carried out. B. Mason, G. A. Fish, S. P. DenBaars, and L. A.Coldren, “Widely Tunable Sampled Grating DBR Laser with IntegratedElectroabsorption Modulator,” Photon. Tech. Letts., 11, (6), 638-640,(June 1999). In such cases the ion-implantation process is notnecessary, but it may be utilized to better tailor other regions such asin integrated modulators and/or amplifier elements. This sequence isfollowed by a regrowth of the upper cladding layers required for the topportion of the optical waveguide.

According to aspects of the present invention, the data signal isavailable in electrical form for monitoring, tapping, and modification.In particular, a packet address or header information can be read andused to determine the routing of the information either by selection ofthe output wavelength or by setting the state of some switch that mightfollow the wavelength converter assembly. These and other desirablefeatures are all incorporated within novel, monolithically-integratedoptoelectronic wavelength converter assembly structures that make use ofa manufacturable, integrated photonic IC technology. G. A. Fish, B.Mason, L. A. Coldren, and S. P. DenBaars, “Compact 1.55 μm Spot-SizeConverters for Photonic Integrated Circuits,” Integrated PhotonicsResearch '99, Santa Barbara, Calif., paper no. RWD4, 375-377, (July19-21, 1999).

Further features of wavelength converter assembly 10 include but are notlimited to,: 1.) providing higher data bandwidths than currentlyavailable from currently available devices (T. Ido, S. Tanaka, M.Suzuki, M. Koizumi, H. Sano, and H. Inoue, “Ultra-High-SpeedMultiple-Quantum-Well Electro-Absorption Optical Modulators withIntegrated Waveguides,” J. Lightwave Techn., 14, (9), 2026-2034,(September 1996)), 2) providing a wider range of possible outputwavelengths than currently available devices (V. Jayaraman, A. Mathur,L. A. Coldren and P. D. Dapkus, “Theory, Design, and Performance ofExtended Tuning Range in Sampled Grating DBR Lasers,” IEEE J. QuantumElec., 29, (6), 1824-1834, (June 1993)), 3.) providing equal or largeroutput optical signal amplitude than current devices without the needfor integrated transistors for electronic amplification (J. K. Kim, E.Hall, O. Sjölund, and L. A. Coldren, “Epitaxially-StackedMultiple-Active-Region 1.55 μm Lasers for Increased DifferentialEfficiency,” Appl. Phys. Letts., 74, (22) 3251-3253, (May 31, 1999); J.T. Getty, O. Buchinsky, R. A. Slavatore, B. Mason, P. G. Piva, S.Charbonneau, K. S. Grabowski, and L. A. Coldren, “MonolithicSeries-Connected 1.55 μm Segmented-Ridge Lasers,” Electronics Letters,35, (15), 1257-1258, (July 22, 1999)), 4.) providing conditioned outputdata waveforms having lower noise and distortion than at the input and5.) providing capabilities for extensions to large arrays of suchwavelength converters, the arrays integrated on one substrate withexisting photonic IC technology. Coldren, L., “Diode Lasers and PhotonicIntegrated Circuits,” Wiley, (1995).

FIG. 1 illustrates certain generic elements, in block diagram form, ofwavelength converter assembly 10. Illustrated are a multisection tunablelaser element 12 (hereafter referred to as “laser 12”), a photodetectorelement 14 (hereafter referred to as “photodetector 14” and a currentconditioning circuit element 16. The insets in the blocks are suggestiveof the possible contents of elements 12, 14 and 16. Current fromphotodetector 14 modulates the laser 12 after being conditioned by theconditioning circuit. The net functionality provides wavelengthconversion of an optical carrier modulated with some data such that: i)an arbitrary output wavelength within a band can be emitted; ii) theamplitude of the output can be adjusted within a useful range; and, iii)the noise and distortion on the data can be reduced. An important aspectof the invention is integration with a common photonic IC technologythat has been described in F. Koyama and K. Iga, “Frequency Chirping inExternal Modulators,” J. Lightwave Tech., 6 (1), 87-93, (January 1998);B. Mason, G. A. Fish, S. P. DenBaars, and L. A. Coldren, “Widely TunableSampled Grating DBR Laser with Integrated Electroabsorption Modulator,”Photon. Tech. Letts., 11, (6), 638-640, (June 1999). Moreover,integration of elements 12, 14 and 16 provides an advantageousfunctionality that is not possible by interconnecting discrete elementsusing conventional printed circuit board or multi-chip moduletechnology. Additionally, integration of elements 12, 14 and 16 enableslow-cost, high-yield manufacturing processes to used.

As illustrated in FIG. 1, laser 12 can include first and second SGDBR's18 and 20, a first and second SOA's 22 and 24 and EAM 26 and a multipleactive region, MAR 28. Photodetector element 16 can include an SOA 30,first and second filters 32 and 34 and an absorber 36.

Wavelength converter 10 offers a number of advantages. In the embodimentillustrated in FIG. 2(a), the surface-illuminated geometry photodetector14 enables efficient and polarization independent coupling of light fromoptical fibers to absorber 36 of photodiode 12. Moreover. it does notrequire a large footprint on the substrate. and its modest dimensions,in one embodiment approximately 10-30 μm in diameter, provide for highbandwidth, sensitive operation. In various embodiments, the sensitivityof wavelength converter assembly 10 can be enhanced by incorporation ofa multi-layer reflective stack beneath the wavelength converter assembly10 to create a resonant-cavity photodiode 14. This stack forms the lowercladding region of laser 12 without any complication. As in otherembodiments, surface-illuminated photodiode 14 is isolated by a protonand/or He+ ion implantation or other means well known to those skilledin the art, rendering the surrounding areas semi-insulating. Inparticular embodiments, the bottom contact of wavelength converterassembly 10 is brought out to the side for biasing and the top contactis directly interconnected to laser 12 with a shunt branch toconditioning circuit 16.

A waveguide layer structure of photodetector 14 illustrated is FIG. 2(b)is identical to the gain section of laser 12. In this embodiment, thewaveguide layer structure of photodetector 14 provides for highersaturation power than typical surface-illuminated designs. Opticalcoupling to the waveguide can be enhanced by the integration ofcompatible mode transformers using techniques such as those described inG. A. Fish, B. Mason, L. A. Coldren, and S. P. DenBaars, “Compact 1.55μm Spot-Size Converters for Photonic Integrated Circuits,” IntegratedPhotonics Research '99, Santa Barbara, Calif., paper no. RWD4, 375-377,(July 19-21, 1999). For ultra high bandwidth embodiments, such as forexample greater than 50 GHz, a terminated traveling wave electrodestructure may be incorporated. A suitable traveling wave electrodestructure is described in 8. T. Ido, S. Tanaka, M. Suzuki, M. Koizumi,H. Sano, and H. Inoue, “Ultra-High-Speed Multiple-Quantum-WellElectro-Absorption Optical Modulators with Integrated Waveguides,” J.Lightwave Techn., 14, (9), 2026-2034, (September 1996).

Referring now to FIG. 2(c), the addition of an optical preamplifier, SOA30, increases the optical signal incident on absorber section 36 andprovides higher output photocurrent. This is advantageous by allowingthe use of low-level data while still obtaining sufficient current toproperly modulate laser 12 and also allows for data regeneration by ashunt conditioning circuit. SOA 30 can also provide for signal leveladjustment in conjunction with an external control circuit. Noise addedby SOA 30 may be removed by current conditioning circuit 18, resultingin a noise figure that does not degrade the data. The layer structure ofSOA 30 can be identical to the gain section of laser 12.

If the signal to noise level is low, or if unwanted data at anotherwavelength is present in the input lightwave, or if a shorter absorbersection is desired, it may be advantageous to place absorber 36 within aresonant cavity to filter out unwanted signals out-of-band. Asillustrated in FIG. 2(d), photodetector 14 is integrated with SOA 30 anda tunable resonant-cavity filter. This waveguide geometry reflects lightsignals that are not within the resonant bandwidth of the resonantcavity formed by two DBR's 38 and 40 and enhances the signal. A shorterabsorber length may be used for total absorption and high quantumefficiency. This shortened length, in turn, reduces photodetector's 14capacitance, enabling very high bandwidth operation.

FIGS. 3(a) and 3(b) are cross-sectional views of the semiconductor layerwaveguide structure of the FIG. 2(d) photodetector 14. In FIG. 3(a)passive sections are created by removal of the active regions prior toregrowth. In FIG. 3(b) passive sections are created by variablethickness and composition quantum-wells via intermixing after uniformgrowth or selective area growth. FIG. 3(a) and 3(b) illustrate thatwaveguide photodetectors 14 are compatible with the tunable sections oflaser 12 that are illustrated in FIG. 5. It will be appreciated thatvarious sections shown in FIGS. 3(a) and 3(b) are omitted in the FIG.2(a), 2(b) and 2(c) embodiments.

FIGS. 4(a) and 5(a) illustrate embodiments of wavelength converterassembly 10 with a series-connected, axially segmented active regionthat obtains signal gain within a widely tunable SGDBR laser 12 asdescribed in U.S. Pat. No. 4,896,325. The principle of operation of eachSGDBR 18 and 20 is well known to those skilled in the art, as is theconcept of using MAR 28 within a single optical cavity to obtain adifferential efficiency greater than unity.

FIGS. 4(b) and 5(b) illustrate another embodiment employing the sameconcepts. In this embodiment, the separate pin active regions of thegain section are integrated vertically with the series electricalconnections derived from intermediate n⁺-p⁺tunnel diodes. This layerstructure is particularly useful in combination with the verticalresonant-cavity photodiode embodiment of FIG. 2(a), since moreabsorption can lead to photodiodes with broader optical bandwidth andbetter efficiency as well. Absorbers can be placed at standing wavepeaks and the tunnel diodes at standing wave nulls to provide amultiplication in absorbency by nearly 2× the number of active regions.

FIG. 4(c) illustrates another embodiment of the invention. In FIG. 4(c),the signal gain is enhanced relative to other embodiments by theaddition of integrated SOA 22 external to the laser cavity. The datasignal current is still applied to the gain section, and the gainsection may either be of a conventional single active region, or MAR 28,as in FIG. 4(a) or 4(b) embodiments, for more signal gain. External SOA22 can provide about 20 dB of gain, whereas the multiple active regiondesign provides for roughly unity gain. Normal lasers have differentialefficiencies ˜20-30%; thus the MAR 28 design gives about 3 to 5×enhancement. The MAR 28 design is advantageous because it does notdegrade the signal-to-noise ratio, whereas SOA 22 does. However, if thesignal level is already high, as it can be with a MAR gain sectionand/or gain in photodetector 14, the constant noise added by SOA 22 canbe negligible. This geometry also allows for the leveling of the outputdata signal level via an external control circuit.

If the signal bandwidth is very high, such as but not limited to 15 GHzor greater, direct modulation of laser 12 may be difficult. Also, ifchirping of the wavelength is a problem, direct modulation may not be anoptimal solution. Finally, if the output wavelength of laser 12 must beset very accurately, direct modulation may compromise the ability ofcontrol circuit 16 to hold the wavelength with sufficient accuracy. Forall of these reasons, use of external modulation, such as illustrated inFIG. 4(d), may be desired. In this case, the data signal current isapplied to the integrated external modulator. An EAM 44 is shown, but aninterferometric modulator, such as a Mach-Zehender or directionalcoupler modulator or other equivalent, is also possible by using passivewaveguide sections of appropriate design. As indicated, SOA's 22 and 24may also be advantageously employed to increase the input carrier leveland output modulated data. Use of MAR 28 actives may also beadvantageous if laser RIN is to be minimized. Since the active regioncan be biased by a high-impedance source in this case, nolow-source-impedance high-frequency signal, the inherent noise on laser12 output can be reduced to sub-shot noise levels. SOA's 22 and 24 atlaser's 12 output can be avoided by accomplishing the desired signalgain in photodetector 14 where their noise may be removed by the currentconditioning circuit. This provides for signal gain, with a maximalsignal-to-noise ratio.

In many embodiments, the current conditioning circuit 16 can be easilycreated in Si-CMOS if external shunting circuits are used. However, forhigh-speed operation, the packaging may not provide sufficiently lowshunt capacitance, so at least some of the functionality may bedesirable to have on-chip. The shunt impedance of this circuit is shownin FIG. 7. With this circuit shunting the drive current, noise on thebaseline (logical ‘0’) and maximum (logical ‘1’) of the data can beremoved, provided that the signal level can be adjusted to theappropriate levels by the gain components in photodetector 14.

One embodiment of an integrable conditioning circuit 16 for the laserbias is shown in FIG. 6. Diode chains can be used to threshold and limitthe level of the modulating data signal. These can be integrated usingthe same fabrication steps already necessary to create photodetector 14and tunable elements of laser 12 shown in FIGS. 2 through 5.

Other conditioning circuits are possible that provide the characteristicof FIG. 7 and the desired laser active region (gain) or EAM bias usingcompatible integrable technology, and these can be obtained by usingstandard circuit design packages. If the photocurrent is to be appliedto the EAM, such as may be desired for high-speed operation, thencurrent conditioning circuit 16 may supply the correct reverse biasvoltage to the EAM for some desired operation. Such desired operationsinclude but are not limited to minimizing the chirp or maximizing thelinearity for an output wavelength from laser 12.

Wavelength converter assembly 10 is a monolithically integratedopto-electronic wavelength converter assembly. Particular embodimentscomprise: photodetector 14 electrically coupled to a multi-section,laser 12 having a differential efficiency greater than unity, where thephotocurrent can be conditioned by a circuit element to provide tapping,thresholding, and limiting of the detected data. Key elements of circuitconditioning circuit 16 can be integrable with the same fabricationsteps required for photodetector 14 and laser 12.

In certain embodiments, photodetector 14 is an edge-illuminatedwaveguide photodetector. In other embodiments, photodetector 14 is asurface-illuminated element. In the waveguide embodiments, SOA 30 may beintegrated with photodetector 14 using the same fabrication sequence foradditional gain or level control. In these embodiments, tunablewaveguide filter 42 may also be incorporated with the same fabricationsequence to filter out unwanted signals or noise from SOA 30.

Laser 12 can use SGDBR's 18 and 20 and gain and phase-shift sections toprovide for output wavelength tunability over a range of several tens ofnanometers. To accomplish net signal gain, the gain section of laser 12may contain several active regions that are driven electrically inseries, and/or laser 12 may incorporate an integrated external SOA atits output port.

In a particular embodiment, the conditioned photocurrent is connected toan integrated external modulator to provide reduced wavelength chirpingand generally enable higher data rate operation than feasible withdirect modulation of the gain section of laser 12. Electro-absorptionmodulators (EAMs) (Robert G. Walker, “High-Speed III-V SemiconductorIntensity Modulators,” IEEE J. Quant. Electron., 27, (3), 654-667,(March 1991); F. Koyama and K. Iga, “Frequency Chirping in ExternalModulators,” J. Lightwave Tech., 6 (1), 87-93, (January 1998); B. Mason,G. A. Fish, S. P. DenBaars, and L. A. Coldren, “Widely Tunable SampledGrating DBR Laser with Integrated Electroabsorption Modulator,” Photon.Tech. Letts., 11, (6), 638-640, (June 1999)) or interferometricmodulators can be integrated within the same fabrication sequence as theother elements. In this particular embodiment, SOA sections precedeand/or follow the modulator section to accomplish net signal gain and/ordata level adjustment. In addition, the combined dispersioncharacteristics of the SOAs and EAM can be used to provide a desired netchirp characteristic.

In other embodiments, current conditioning circuit element 16 isnon-linear and consists of a connection to an external source to supplylaser 12 with a necessary threshold current. In another embodiment,current conditioning circuit 16 includes a microwave filter to removesubcarrier header information. In still another embodiment, currentconditioning circuit 16 can comprise a limiting circuit to shunt off anycurrents above a given level. Current conditioning circuit element 16can also comprise a thresholding circuit to shunt away photocurrentbelow a given level. These latter circuits may be partially external tothe monolithic photonic IC, or they may comprise appropriate seriesdiode chains that can be compatibly integrated.

All or some of the elements of wavelength converter assembly 10 can becreated with a standardized photonic IC fabrication processes. Thus,various options can be added dependant only upon the desiredspecifications and without the need to develop a new or incompatiblematerials growth and device fabrication sequence. In one specificembodiment, wavelength converter assembly 10 includes elements that arebased on InP substrates, which can provide wavelength conversion andother functionality near the 1.55 μm wavelength band. It will beappreciated that wavelength converter assembly 10 can use other materialplatforms.

Another embodiment of wavelength converter assembly 10 is illustrated inFIGS. 8(a) and (b). In this embodiment, metal interconnects betweenphotodetector 14 and laser 12 are avoided by integrating photodetector14 directly on top of laser 12. This eliminates any excess seriesresistance or inductance or shunt capacitance between the input andoutput stages and optimizes the configuration for high-data rateoperation. Semi-insulating regrowth of a buried-heterostructurewaveguide is also illustrated for high-speed operation. Currentconditioning circuit 16 can also be connected by contacting to theintermediate p-InGaAsP layer between vertically stacked photodetector 14and laser 12. As illustrated, the connection is directly to theintegrated modulator, which can be the preferred connection forhigh-speed low-chirp operation. Vertical illumination is alsoillustrated, but a horizontal waveguide detector configuration is alsopossible. The vertical configuration may be preferred since there isless crosstalk between input and output lightwave signals. Such verticalintegration is obtained by performing several regrowth steps as iscommon in such photonic integrated circuits using techniques well knownto those skilled in the art.

In one mode of operation of the FIGS. 8(a) and (b) embodiment, a reversebias voltage is applied between bias-1 and bias-2 electrodes to depletethe InGaAs absorber region and provide minimal sweep out times forphotocarriers. Bias-2 would is set to the voltage appropriate foroptimal operation of the modulator. Example dc potentials include butare not limited to, −2 V on bias-2 electrode and −6 V on bias-1electrode. The thickness of the InGaAs absorber is adjusted to besufficient to absorb most of the incoming light but not so thick as toslow the transit of carriers to the contact layers. It will beappreciated that an avalanche photodetector (APD) may also be used inplace of the simple pin detector indicated in FIGS. 8(a) and (b). Inthis case additional layers are desired to optimize the gain-bandwidthproduct of the APD.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

What is claimed is:
 1. A method of converting an optical wavelength,comprising: providing a wavelength converter assembly with aphotodetector and a laser with a common epitaxial structure with areasof differing bandgap, the laser including a laser resonator; absorbingan optical input having a first wavelength at the wavelength converterassembly; generating a first electrical signal from the photodetector inresponse to the optical input; conditioning the first electrical signaland produce a conditioned first electrical signal; generating a secondelectrical signal from the conditioned first electrical signal;generating a laser output from a gain medium of the laser at a secondwavelength in response to the second electrical signal.
 2. The method ofclaim 1, wherein the wavelength converter assembly includes a currentconditioning circuit coupled to the photodetector and the laser.
 3. Themethod of claim 2, wherein the current conditioning circuit conditionsthe first electrical signal and produces the conditioned firstelectrical signal.
 4. The method of claim 3, wherein the conditioningcircuit contains nonlinear circuit elements to limit a magnitude of thesecond electrical signal wherein a maxima of signal pulses are reshaped.5. The method of claim 3, wherein the conditioning circuit containsnonlinear circuit elements to supply a minimum level to the secondelectrical signal wherein a minima of signal pulses are reshaped.
 6. Themethod of claim 5, wherein the substrate is semi-insulating InP.
 7. Themethod of claim 3, further comprising: supplying a minimum level to thesecond electrical signal; and reshaping a minima of signal pulses. 8.The method of claim 3, wherein the current conditioning circuit providesbias voltages to the photodetector and laser.
 9. The method of claim 1,wherein conditioning the first electrical signal includes modifying thefirst electrical signal's bias and threshold.
 10. The method of claim 1,wherein the optical input is absorbed at the photodetector and a gainfrom the gain medium occur at the common epitaxial structure.
 11. Themethod of claim 1, wherein the optical input is absorbed at thephotodetector and a gain from the gain medium occur at a common level ofthe common epitaxial structure.
 12. The method of claim 1, furthercomprising: controlling a phase of the laser output.
 13. The method ofclaim 1, wherein the laser has a first reflector and a second reflectorthat define the laser resonator, wherein at least one of the first andsecond reflectors is frequency selective and tunable.
 14. The method ofclaim 13, further comprising: tuning a maximum reflectivity of the firstreflector relative to a maximum reflectivity of the second reflector toselect the second wavelength.
 15. The method of claim 1, furthercomprising: modulating the laser output.
 16. The method of claim 1,wherein the second electrical signal modulates the laser's output. 17.The method of claim 1, further comprising: selecting a range ofwavelengths for detection at the photodetector.
 18. The method of claim1, further comprising: amplifying the optical input prior to absorptionby the wavelength converter assembly.
 19. The method of claim 1, whereinthe second electrical signal modulates the laser's output.
 20. A methodof converting an optical wavelength, comprising: providing a wavelengthconverter assembly having an epitaxial structure with areas of differingbandgap that includes a waveguide layer positioned between first andsecond semiconductor layers of the epitaxial structure, an opticallyactive gain medium positioned between first and second reflectors thatdefine a resonant cavity, and a photodetector; detecting an opticalinput at the photodetector; and generating a laser output from thewavelength converter assembly in response to the optical input.
 21. Themethod of claim 20, wherein the laser output is modulated outside thelaser resonator at the common epitaxial structure.
 22. The method ofclaim 20, wherein the optical input has a first wavelength.
 23. Themethod of claim 22, further comprising: generating a first electricalsignal from the photodetector in response to the optical input.
 24. Themethod of claim 23, further comprising: conditioning the firstelectrical signal and produce a conditioned first electrical signal. 25.The method of claim 24, wherein the wavelength converter assemblyincludes a current conditioning circuit coupled to the photodetector andthe laser.
 26. The method of claim 25, wherein the current conditioningcircuit conditions the first electrical signal and produces theconditioned first electrical signal.
 27. The method of claim 24, furthercomprising: generating a second electrical signal from the conditionedfirst electrical signal.
 28. The method of claim 27, further comprising:providing bias voltages to the photodetector and laser.
 29. The methodof claim 27, further comprising: limiting a magnitude of the secondelectrical signal; and reshaping a maxima of signal pulses.
 30. Themethod of claim 27, further comprising: generating the laser output froma gain medium of the laser at a second wavelength in response to thesecond electrical signal.
 31. The method of claim 20, furthercomprising: controlling a phase of the laser output.
 32. The method ofclaim 20, further comprising: tuning a maximum reflectivity of the firstreflector relative to a maximum reflectivity of the second reflector toselect the laser output.
 33. The method of claim 20, further comprising:modulating the laser output.
 34. The method of claim 33, wherein thelaser output is modulated outside the resonant cavity at the commonepitaxial structure.
 35. The method of claim 20, further comprising:selecting a range of wavelengths for detection at the photodetector. 36.The method of claim 20, further comprising: amplifying the optical inputprior to prior to detecting the optical input.